The present invention relates generally to devices for providing a point of contact indicating target on the surface of a work piece, and more particularly to a novel method and device for providing a precise target point on a work surface at a position representing the working position of a tool when moved in a predetermined path from a position spaced from the work piece to a working position contacting the work piece.
With the advent of microelectronics and the rapid development of microprocessors employing integrated circuits and microelectronic chips or wafers, apparatus has been developed to facilitate design, development, analysis and assembly of the microelectronic chips, and particularly apparatus which enables bonding of a conductor wire to a conductive element on a microelectronic chip for connection to either a second point located within the boundary of the chip or a point external to the chip such as on a carrier board or lead frame. Such apparatus are generally termed wire bonders.
Wire bonding is basically a welding operation in which a weldment is formed between a conductive bonding wire or ribbon and a metallic conductive element or pad on the microelectronic chip or waver. The known wire bonders accomplish bonding generally in accordance with one of three basic principles; thermocompression ball bondings, thermosonic ball bonding and ultrasonic wedge bonding. In general, the wire bonder apparatus for carrying out any one of the three techniques includes a work support station enabling support of a work piece such as a microelectronic chip, a supply of generally continuous length bonding wire or ribbon such as a supported spool or reel of bonding wire, a bonding head having a bonding tool adapted for cooperation with the bonding wire and operable in a manner to effect engagement between a contact end or tail of the bonding wire and the work surface, and viewing means generally in the form of a microscope enabling magnified viewing of the work surface so as to enable accurate positioning of the work piece in relation to the bonding tool to effect a bonding connection at a predetermined desired location on the work piece.
To assist in viewing the work piece and enable accurate positioning thereof relative to the path traversed by the bonding tool as it is moved from a position spaced from the work piece to a position engaging the work piece during a bond connection, it is a common practice to provide illumination to the work piece in the form of a light source adapted to direct a white light onto the work piece. A significant problem exists with such known apparatus in that the sight line of the viewing microscope is generally inclined relative to a line perpendicular to the work surface so that establishing alignment between a predetermined position on the work piece and the point at which the bonding tool will make contact with the work piece is very difficult, particularly with the bonding tool spaced above the plane of the work surface as in a search level enabling the operator to move the work piece relative to the bonding tool without engaging the bonding tool.
In an attempt to both reduce the time required for an operator to accurately position a work piece relative to a bonding tool in the known wire bonder apparatus, point of contact indicating devices have been developed which are adapted to provide a target spot on a work surface, such as on a microelectronic chip, to enable the bonder operator to more readily align the work piece precisely beneath the bonding tool. In one known technique for indicating point of contact, a simple cross-line reticle is mounted in the fixed eyepiece of a microscope and is superimposed over the image of the work piece. Another technique employs an illuminated cross-line or other pattern which is directed into the optical system of the microscope and is also superimposed over the image of the workpiece. The known techniques or systems have the disadvantage that they are dependent upon the optical and mechanical stability of the microscope. Further, since virtually all microscopes have an image shift when focused or zoomed to a different magnification, any disturbance of the microscope generally requires that the cross-line be recalibrated.
More recent attempts at providing improved and more accurate point of contact indication on a work piece have employed incandescent lamps as light sources from which a target is projected onto the work surface to indicate point of contact. These devices have proven to be of only marginal usefulness due to the fact that the target spot is very dim and thus makes it very difficult for the operator to see the target spot, particularly where a white light illumination beam is being employed to illuminate the work piece surface beneath the viewing aid. Although incandescent lamps can be made bright enough that they can cause eye damage, they are very inefficient at producing a colored point of light for a number of reasons. For example, the radiation emitted by an incandescent lamp is emitted into a sphere and only a fraction of the useful light can be collected. Another reason is that the source of light in an incandescent lamp is a tungsten filament much larger than the final target spot desired so that substantial losses are incurred in optically reducing the size of the source. Still further, an incandescent lamp light source must be colored to produce a contrast with the white light generally emitted by microscope illuminators. Such coloring of incandescent light sources is conventionally done with optical filters which only transmit a small percentage of the visible radiation. Finally, of the radiation incident upon a work piece from an incandescent point of contact indicating device, only a small fraction is collected by the microscope objective lens because the target source light is scattered when it hits the work piece surface.